1. Field of the Invention
The present invention mainly relates to a small bearing serving both as a sliding and a rolling bearing.
2. Description of the Related Art
Generally, the load bearing mechanism of a rolling bearing includes an inner and an outer races, and a plurality of balls which move to roll between the both races. Typical examples of such a rolling bearing are a ball bearing as shown in FIG. 8 and a cylindrical roller bearing as shown in FIG. 9.
In a needle roller bearing, needle rollers move to roll between an inner and an outer races, or between an outer race and a shaft. Typical examples of the needle roller bearing are a shell-type needle roller bearing as shown in FIG. 10 and a solid-type needle roller bearing having an inner and an outer races as shown in FIG. 11.
In a sliding bearing, a relative sliding movement is performed between the bearing surface for bearing load and the shaft. A bush-type sliding bearing as shown in FIG. 12 is an example of this type of bearing.
In well-known rolling bearings, steel balls or needle rollers bear load in rolling contact while rotating and revolving. In producing such rolling bearings, the outer and inner races and the needle rollers are formed by machining and then subjected to processes such as quenching and polishing. Thus, a lot of processes and skills of experts are required, resulting in high production costs. Due to its mechanism, a rolling bearing develops a gap between its members when a wear loss resulting from the rolling contact has progressed to a certain degree, which leads to an increase in noise and a deterioration in bearing functions.
Even in cases where the bearing is required to be light and thin, a needle roller bearing, though poorer in function as compared to a sliding bearing, is adopted where bearing performances for medium and high-speed operations are required. That is, in the operational range of low load (at a bearing pressure ranging from 1 to 50 kgf/cm.sup.2) and of medium and high speeds (at a peripheral speed ranging from 30 to 100 m/min.), needle roller bearings are being mostly adopted. When compared with a sliding bearing, a needle roller bearing has a larger wall thickness and a larger weight. Further, since its structure is the same for both large and small-load types, it is difficult to make its wall thickness as thin as in the case of a sliding bearing.
A shell-type needle roller bearing has a smaller wall thickness as compared to a solid-type needle roller bearing. However, it is difficult to reduce its wall thickness to the level of a sliding bearing. Although the wall thickness of a shell-type needle roller bearing can be made smaller than that of a solid-type needle roller bearing, it is necessary, due to the load transmission between the shaft and the rollers, to conduct finish processes, such as quenching and polishing are required, on the shaft, resulting in high production costs. Further, in long term operations, the problem of wear of the shaft is also involved.
In the case of small-diameter bearings, sliding bearings are generally lighter in weight as compared to rolling bearings. However, when used for medium and high-speed operations (at a peripheral speed ranging from 30 to 100 m/min. or higher), the sliding bearings require a lubricant (oil), which usually makes them rather expensive due to provision of the lubricant, lubrication method, etc. When a sliding bearing is used without lubrication, the friction generated is greater than in the case of a rolling bearing, so that a large driving force is required of the equipment concerned. It is true that a non-lubrication-type sliding bearing is advantageous in terms of weight and thickness as compared to a needle roller bearing. However, it is much poorer than the latter in terms of coefficient of friction. While the coefficient of friction (.mu.) of a needle roller bearing ranges from 0.002 to 0.010, that of a non-lubrication-type sliding bearing (e.g., one of PTFE containing a filler material) ranges from 0.10 to 0.30. Thus, in the case of a non-lubrication-type sliding bearing, the equipment concerned must have a large driving force. Thus, for apparatuses which are intended to be smaller in size and weight, the large drive source and the large equipment weight constitute inhibiting factors. Further, it has conventionally been difficult to adapt a non-lubrication-type sliding bearing to medium-speed specifications (where the peripheral bearing speed ranges from 30 to 100 m/min.). Further, the relative sliding movement between the shaft and the bearing causes wearing thereof to generate wear products of particles. Where a mechanism is required for preventing the generation of such wear products, designing of bearings is often difficult.